Mountain pass

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A mountain pass as it appears on a contour map: Bwlch Maesgwm in Snowdonia, north Wales, United Kingdom.<ref>Template:Coord, height contours from SRTM data.</ref>

A mountain pass is a navigable route through a mountain range or over a ridge. Since mountain ranges can present formidable barriers to travel, passes have played a key role in trade, war, and both human and animal migration throughout history. At lower elevations it may be called a hill pass. A mountain pass is typically formed between two volcanic peaks or created by erosion from water or wind.

Overview

Idealised mountain pass represented as the green line; the saddle point is in red.

Mountain passes make use of a gap, saddle, col or notch. A topographic saddle is analogous to the mathematical concept of a saddle surface, with a saddle point marking the minimum high point between two valleys and the lowest point along a ridge.Template:Sfn Template:Sfn On a topographic map, passes can be identified by contour lines with an hourglass shape, which indicates a low spot between two higher points.Template:Sfn In the high mountains, a difference of Template:Convert between the summit and the mountainTemplate:Clarify is defined as a mountain pass.Template:Sfn

Passes are often found just above the source of a river, constituting a drainage divide. A pass may be very short, consisting of steep slopes to the top of the pass, or a valley many kilometers long, whose highest point might only be identifiable by surveying.

Roads and railways have long been built through passes. Some high and rugged passes may have tunnels bored underneath a nearby mountainside, as with the Eisenhower Tunnel bypassing Loveland Pass in the Rockies, to allow faster traffic flow throughout the year.

The top of a pass is frequently the only flat ground in the area, and may be a high vantage point. In some cases this makes it a preferred site for buildings. If a national border follows the ridge of a mountain range, a pass over the mountains is typically on the border, and there may be a border control or customs station, and possibly a military post. For instance, Argentina and Chile share the world's third-longest international border, Template:Convert long, which runs north–south along the Andes mountains and includes 42 mountain passes.<ref name="pasos" /><ref name="pasos_chile" />

Apart from offering relatively easy travel between valleys, passes also provide a route between two mountain tops with a minimum of descent. As a result, it is common for tracks to meet at a pass; this often makes them convenient routes even when travelling between a summit and the valley floor. Passes traditionally were places for trade routes, communications, cultural exchange, military expeditions etc. A typical example is the Brenner Pass in the Alps.

Some mountain passes above the tree line have problems with snow drift in the winter. This might be alleviated by building the road a few meters above the ground, which will make snow blow off the road.

Synonyms

There are many words for pass in the English-speaking world. In the United States, pass is very common in the West, the word gap is common in the southern Appalachians, and notch is seen in many parts of New England. The term col, derived from Old French, is also used, particularly in Europe.Template:Citation needed

In the highest mountain range in the world, the Himalayas, passes are denoted by the suffix "La" in Tibetan, Ladhakhi, and several other regional languages. Examples are the Taglang La at 5,328 m (17,480 ft) on the Leh-Manali highway, and the Sia La at 5,589 m (18,337 ft) in the Eastern Karakoram range.

Scotland has the Gaelic term bealach (anglicised "balloch"), while Wales has the similar bwlch (both being insular Celtic languages).<ref name="Nicolson 1945 p. 28">Template:Cite book</ref><ref name="The Geology of Snowdonia – A Collection of Historical Articles on the Physical Features of the Peaks of Snowdonia 2013 p. 45">Template:Cite book</ref> In the Lake District of north-west England, the term hause is often used, although the term pass is also common—one distinction is that a pass can refer to a route, as well as the highest part thereof, while a hause is simply that highest part, often flattened somewhat into a high-level plateau.

In Japan they are known as tōge, which means "pass" in Japanese. The word can also refer to narrow, winding roads that can be found in and around mountains and geographically similar areas, or specifically to a style of street racing which may take place on these roads.

Around the world

There are thousands of named passes around the world, some of which are well-known, such as the Khyber Pass close to the present-day Afghanistan-Pakistan border on the ancient Silk Road, the Great St. Bernard Pass at Template:Convert in the Alps, the Chang La at Template:Convert, the Khardung La at Template:Convert in Ladakh, India and the Palakkad Gap at Template:Convert in Palakkad, Kerala, India, which is the widest mountain pass in the world.Template:Cn The roads at Mana Pass at Template:Convert and Marsimik La at Template:Convert, on and near the China–India border respectively, appear to be world's two highest motorable passes. Khunjerab Pass between Pakistan and China at Template:Convert is also a high-altitude motorable mountain pass. One of the famous but non-motorable mountain passes is Thorong La at Template:Convert in Annapurna Conservation Area, Nepal.